Methyl Iodide—An Interesting Reagent for Positive Ion Chemical Ionization

Abstract

Methyl iodide (CH3I) has been used as a chemical ionization reagent gas and the ion/molecule reactions of formed reagents ions with a number of nucleophiles were studied in a chemical ionization source of a conventional mass spectrometer. Generalizing, charge exchange reactions dominate for compounds with lower ionization energy than CH3I, while the others preferred protonation and methylation reactions. For ketones both protonation and charge exchange reactions were thermodynamically possible and the prevailing reaction was dependent on the compound. Surprisingly, all the isomeric C5 ketones gave rise to clearly different spectra. Alcohols, having higher ionization energies than CH3I and a lower proton affinity than that of CH2I., also reacted interestingly. Ionization through hydroxide abstraction prevailed for C3–C6 alkanols. The formation of direct adduct ions was rare but, in contrast, substitution product ions [M − OH + CH3I]+ and [M − H2O + I]+ were observed for alkanols. © 1997 John Wiley & Sons, Ltd.